1. Field of the Invention
This invention relates to an optical head and disk apparatus which use near field wave, and a method for manufacturing optical heads, and more particularly, relates to an optical head which implements high density recording on a recording medium and a small-sized optical head of improved data transfer rate, a disk apparatus, a method for manufacturing optical heads, and an optical element for an optical head.
2. Description of Related Art
In the field of optical disk apparatus, the optical disk has changed historically from the compact disk (CD) to the digital video disk (DVD), which has a large recording capacity and is capable of high density recording. The recent development of high performance computers and high resolution displays has resulted in increasing demand for large capacity recording.
The recording density of an optical disk depends basically on the diameter of an optical spot formed on a recording medium. Recently, the near field wave technology in the field of the microscope has been applied to the optical recording technology as a technology for miniaturizing the beam spot diameter. As the conventional optical disk apparatus which uses the near field wave, for example, the optical disk described in the literature (Jpn. J. Appl. Phys., Vol. 35 (1996) P. 443) and U.S. Pat. No. 5,497,359 has been known.
FIG. 21(a) and FIG. 21(b) show an optical disk apparatus described in the literature (Jpn. J. Appl. Phys., Vol. 35 (1996) P. 443). As shown in FIG. 21(a), the optical disk apparatus 190 is provided with a semiconductor laser 191 that emits a laser beam 191a, a coupling lens 192 that changes the laser beam 191a emitted from the semiconductor laser 191 to a collimated beam 191b, and an optical fiber 193 which is polished in a taper shape having a larger diameter at the incident end 193a and a smaller diameter at the emission end 193b, and provided with a probe 194 that introduces the collimated beam 191b which comes from the coupling lens 192 from the incident end 193a, and a recording medium 195 on which the information is recorded by means of the near field wave 191c that leaks from the emission end 193b of the optical fiber 193.
The recording medium 195 has a recording layer 195a consisting of GeSbTe, which is a phase change recording medium, which recording medium is heated by incident near field wave 191c, and then the heating causes phase change between crystal/amorphous, and difference in reflectance between both phases is utilized for recording.
The optical fiber 193 has the incident end 193a having a diameter of 10 .mu.m and the emission end 193b having a diameter of 50 nm, and is coated with a metal film 194b consisting of a metal such as aluminum with interposition of a clad 194a to prevent the beam from leaking to somewhere other than the emission end 193b. The diameter of the near field wave 191c has the approximately same diameter as the diameter of the emission end 193b, therefore the high density recording of several 10 Gbits/inch.sup.2 is possible.
For reproduction, as shown in FIG. 21(b), a near field wave 191c having such a low power as it does not cause phase change is irradiated onto the recording layer 195a by use of the same optical head as used for recording, the reflected beam 191d from the recording layer 195a is condensed on a photomultiplier 197 by means of a condenser lens.
FIG. 22 shows an optical head of an optical disk apparatus disclosed in U.S. Pat. No. 5,497,359. The optical head 50 is provided with an condense lens 52 that condenses a collimated beam 51 and an Super SIL (Super Solid Immersion Lens) 54 having the form of bottom-cut sphere placed with the bottom plane 54a perpendicular to the condensed beam 53 from the condense lens 52. The collimated beam 51 is condensed by the condense lens 52 and the condensed beam 53 is incident onto the spherical incident surface 54b, the condensed beam 53 is refracted at the incident surface 54b and condensed on the bottom surface 54a to form a beam spot 55 on the bottom surface 54a. Because the wavelength of the beam becomes short in inversely proportional to the refractive index in the internal of the super SIL 54, the diameter of the beam spot becomes small in proportion to it. A part of the beam condensed on the beam spot 55 is totally reflected toward the incident surface 54b, but the beam leaks partially from the beam spot 55 to the outside of the super SIL 54 as a near field wave 57. A recording medium having the approximately same refractive index as that of the super SIL 54 is located at the close distance from the bottom surface 54a so that the distance is sufficiently smaller than a wavelength of the wave, then the near field wave 57 is coupled with the recording medium 56 and propagates in the recording medium 56. The information is recorded on the recording medium 56 by the propagation beam.
By structuring the Super SIL 54 so that the collimated beam 51 is condensed at the position r/n (r denotes the radius of the Super SIL) distant from the center 54c of the semi-spherical surface 54b, the spherical aberration due to the Super SIL 54 is reduced and the numerical aperture in the Super SIL 54 is increased, and further the diameter of the beam spot 55 is minimized. In detail, the beam spot 55 is minimized according to the equation 1. EQU D.sub.1/2 =k.lambda./(n.multidot.NAi)=k.lambda./(n.sup.2.multidot.NAo) (1)
where,
D.sub.1/2 : beam spot diameter where the intensity becomes a half of the maximum intensity. PA1 k: proportional constant (normally around 0.5) which depends on the intensity distribution of an optical beam PA1 .lambda.: wavelength of an optical beam PA1 n: refractive index of an Super SIL 54 PA1 NAi: numerical aperture in an Super SIL 54 PA1 NAo: numerical aperture of an incident beam to an Super SIL 54
The collimated beam 51 is condensed as the beam spot 55 without absorption on the optical path and high optical utilization factor is obtained. As the result, a beam source having a relatively low output is sufficient for use and the reflected beam is detected without a photomultiplier.